1. Field of the Invention
The invention is concerned with a silicon detector for detecting high-intensity radiation particles. The silicon detector used comprises a silicon wafer having an entrance opening in low-resistivity silicon, and a passivation layer of silicon dioxide and a sensitive volume of high-resistivity silicon for converting the radiation particles into detectable charges. Furthermore, the detector comprises electrodes built in the form of vertical channels etched into the sensitive volume for collecting the charges, and read-out electronics for generating electrical signals from the collected charges. The invention is also concerned with a method of manufacturing a radiation detector and the use of the detector for measuring radiation.
2. Description of the Related Art
In experimental and applied particle physics, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify high-energy particles, such as particles produced by nuclear decay, cosmic radiation, or reactions in a particle accelerator. An accelerator is a device (i.e. machine) used to produce such high-energy particles, which are high-speed beams or pulses of charged particles, such as electrons, protons, or heavy ions, for research in high-energy and nuclear physics, synchrotron radiation research, medical therapies, and some industrial applications.
The particle pulses to be detected can be generated by laser-plasma interaction by different plasma shot techniques. In physics and chemistry, plasma is typically an ionized gas. Plasma is considered to be a distinct state of matter, apart from gases, because of its unique properties and is typically formed by heating and ionizing a gas, stripping electrons away from atoms, thereby enabling the positive and negative charges to move more freely.
“Ionized” refers to presence of one or more free electrons, which are not bound to an atom or molecule and ionization is the process by which a neutral atom or molecule acquires a positive or negative charge. The free electric charges make the plasma electrically conductive so that it responds strongly to electromagnetic fields.
Radiation as used in physics, is energy in the form of waves or moving subatomic particles. Radiation can be classified as ionizing or non-ionizing radiation, depending on its effect on atomic matter. Ionizing radiation has enough energy to ionize atoms or molecules while non-ionizing radiation does not. Radioactive material is a physical material that emits ionizing radiation.
Ionizing radiation has enough energy to eject from electrically neutral atoms, leaving behind charged atoms or ions. There are four basic types of ionizing radiation: Alpha particles (helium nuclei), beta particles (electrons), neutrons, and gamma rays (high frequency electromagnetic waves, x-rays, are generally identical to gamma rays except for their place of origin.) Neutrons are not themselves ionizing but their collisions with nuclei lead to the ejection of other charged particles that do cause ionization.
As human sense is unable to react to ionizing radiation, detectors are used to detect its presence. With a detector it is possible to determine the type of radiation, measure its energy and register other parameters. Modern detectors are also used as calorimeters to measure energy of the detected radiation. They may also be used to measure other attributes such as momentum, spin, charge etc. of the particles.
There are dedicated detectors for different type of radiation. To detect radiation, one utilizes the interaction process with matter where the interacting medium converts the invisible radiation to detectable signals. If the radiation consists of charged heavy particles, such as alphas, or light ones, like electrons or positrons, the electromagnetic interaction create charges which can be collected and detected. It can also initiate further processes, which can give rise to registable signals in the detector medium. The neutral gamma radiation interacts with matter with electromagnetic processes and transfer part or all its energy to electrons. To register other neutral particles, like neutrons similar processes are relied on. The radiation has to interact with matter and transfer its energy to charged particles (electrons) and then the detection process mentioned above.
All detectors use the fact that the radiation interacts with matter, mostly via ionization. The detector converts deposited energy of the ionizing radiation to registered signals, usually electric signals. The interaction with the radiation takes place in an interacting medium and creates charges that are collected and detected.
Many of the detectors invented and used so far are ionization detectors (of which gaseous ionization detectors and semiconductor detectors are most typical) and scintillation detectors; but other, completely different principles have also been applied, like Cherenkov light and transition radiation.
The most widely used type of detectors is based on the effect produced when a charged particle passes through a gas. The charged particle will along its track through the gas ionize the gas molecules. The energy needed to create an electron pair in gases is in the order of 30 eV and depends on the type of gas. If an electric field is applied, the created charge will be collected on the electrodes resulting in an electric pulse, which contains the total collected charge and thus the absorbed energy of the electrons.
The functionality of a semiconductor or solid state detector is analogous to gas ionization devices. Since the basic information carriers are the electron hole pairs created along the path of the ionizing particle, the charge per unit length will be higher than in the gaseous detection chamber.
A semiconductor detector is a device that uses a semiconductor (usually silicon or germanium) to detect traversing charged particles or the absorption of photons. The density of the ionizing medium, the semiconductor material, is some 1000 times greater than for a gas. In addition, the energy needed to create an electron-hole pair is ten times lower, i.e. 3,7 eV for Si and 2,9 eV for Ge. In the field of particle physics, these detectors are usually known as silicon detectors. When their sensitive structures are based on a single diode, they are called semiconductor diode detectors. When they contain many diodes with different functions, the more general term semiconductor detector is used. Semiconductor detectors have found broad application during recent decades, in particular for gamma and X-ray spectrometry and as particle detectors.
In these detectors, radiation is measured by means of the number of charge carriers set free in the detector, which is arranged between two electrodes. The number of the free electrons and the holes (electron-hole pairs) produced by the ionizing radiation is proportional to the energy transmitted by the radiation to the semiconductor. As a result, a number of electrons are transferred from the valence band to the conduction band, and an equal number of holes are created in the valence band. Under the influence of an electric field, the electrons and the holes travel to the electrodes, where they result in a pulse that can be measured in an outer circuit. The holes travel into the opposite direction than the electrons and both can be measured. As the amount of energy required to create an electron-hole pair is known, and is independent of the energy of the incident radiation, measuring the number of electron-hole pairs allows the energy of the incident radiation to be found.
The energy required for production of electron-hole-pairs is very low compared to the energy required for production of paired ions in a gas detector. Consequently, in semiconductor detectors, the statistical variation of the pulse height is smaller and the energy resolution is higher. As the electrons travel fast, the time resolution is also very good, and is dependent upon rise time. Compared with gaseous ionization detectors, the density of a semiconductor detector is very high, and charged particles of high energy can give off their energy in a semiconductor of relatively small dimensions.
In microelectronics, a wafer is a thin slice of semiconducting material, such as a silicon crystal, upon which microcircuits are constructed by doping (for example, diffusion or ion implantation), chemical etching, and deposition of various materials. Wafers are thus of key importance in the fabrication of semiconductor devices such as integrated circuits.
Most silicon particle detectors work, in principle, by doping narrow (usually around 100 micrometers wide) strips of silicon to make them into diodes, which are then reverse biased. A diode is a component that restricts the directional flow of charge carriers. Essentially, a diode allows an electric current to flow in one direction, but blocks it in the opposite direction. As charged particles pass through these strips, they cause small ionization currents which can be detected and measured. Arranging thousands of these detectors around a collision point in a particle accelerator can give an accurate picture of what paths particles take.
Silicon detectors have a much higher resolution in tracking charged particles than older technologies such as cloud chambers or wire chambers. The drawback is that silicon detectors are much more expensive than these older technologies and require sophisticated cooling to reduce leakage currents (noise source) as well as suffer degradation over time from radiation.
Also a scintillation counter can measure ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic or organic liquid that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube (PMT) measures the light from the crystal. The PMT is attached to an electronic amplifier and other electronic equipment to count and possibly quantify the amplitude of the signals produced by the photomultiplier 
Scintillation counters are widely used because they can be made inexpensively yet with good quantum efficiency. The quantum efficiency of a gamma-ray detector depends upon the density of electrons in the detector, and certain scintillating materials, such as sodium iodide and cesium iodide activated thallium, achieve high electron densities as a result of the high atomic numbers of some of the elements of which they are composed.
Scintillators often convert a single gamma, ion or a neutron of high energy radiation into high number of lower-energy photons, where the number of photons per unit of absorbed energy is fairly constant. By measuring the intensity of the flash (the number of the photons produced by the incoming radiation) it is therefore possible to discern the original particle's energy.
There are many different types of CsI(Tl) crystals for the different channels of e.g. a Neutral Particle Analyzer (NPA), which is a detector for detection of neutral radiation. The crystals of CsI(Tl) are used as wafer and are optically connected to photomultipliers. They are made in various sizes ranging from 1 inch (25.4 mm) to 11.8 inches (300 mm), and thicknesses of the order of 0.5 mm.
As a result of the increase in the power of the plasma shots used today for producing ionized particles to be detected, these detectors becomes saturated and consequently made them useless for the measuring and neither is the energy resolution good enough to resolute different ion components. These detectors can not operate at higher count rates than 100 kHz, mainly due to the CsI(Tl) scintillation time of about 1 microsecond, i.e. they do not allow to carry out pulse high analysis of ion signals under the intensive gamma and neutron background. The crystals of CsI(Tl) can not even without any damaged operate at a higher count rate (at intense radiation) due to the long scintillation constant of the scintillator itself.
There are two mechanisms interplaying here. Damage cause that the material is not able to work as before and count rate is another problem when the detectors get saturated by the quantity of particles hitting it. These mechanisms are not related to each other. Here, with radiation hardness, in fact radiation resistance to such a field is meant. As a consequence of the radiation damage, the energy resolution is worsened and the amplitude of the interested particles become too small that can not be separated from the background signals, which is called degradation of the signal-to-noise ratio. The reason for the problem arising with is thus that they have a long scintillation time constant, as a consequence of which signals can not be integrated in short time and a low count rate have to be used.
U.S. Pat. No. 5,552,596 is mentioned as prior art. It presents a three-dimensional radiation detection device for detecting X and gamma radiation. An array of electrodes penetrates into the substrate bulk of a thickness of some hundreds microns.
The article “Technology development of 3-D Detectors for High Energy physics and Imaging by Giulio Pellegrini et al (Department of electronics and electrical Engineering, Glasgow University, G128QQ UK presents a tree-dimensional array of electrodes penetrated into the detector bulk. A layer of silicon of about 200 microns thick is used. The advantages of this structure include short collection distances, fats collection times and low depletion voltages depending on the electrode diameter and pitch chosen.
A problem in conventional semiconductor detectors arises when high-intensive radiation such as 1010-1011 neutrons/cm2 is measured because of the intrinsic damage suffers by the silicon bulk of the sensitive volume. The damage correlate with the detector thickness this means for larger thickness damage has larger probability, so in another words damage scales with thickness.
The radiation that hit the detector has time to destroy the detector and induce damages in the sensitive silicon layer, wherein the electrons and holes are intended to be formed. The silicon wafer, however, has to be constructed so that the capacitance would be low enough. Also technological problems have set it limitations to the structure of the wafers. Therefore, such high-intense radiation can not be measured with conventional semiconductor and silicon detectors.
The detection of light ions is very difficult as any material in the input window can reduce the energy, and even stop the incoming ions. Standard radiation detectors work in full depletion regime, and this requires the use of electrodes at both sides of the detecting bulk. Therefore it is needed a metallic electrode, acting as backside contact, in the ion input window. Other authors [Tindall et al.] have proposed the use of ultra-thin metal layers.
U.S. Pat. No. 6,259,099 presents an ultra-thin ionizing radiation detector using such electrical contacts at both sides of the detecting volume. It is necessary to apply a voltage between the two electrodes to have an electrical signal. The metals are at the top and bottom surfaces. The voltage is applied vertically. The use of a metal layer in the entrance window is a hinder for detecting ions with low energy and this is due to the thickness of their entrance window.
A second problem with conventional silicon detectors and semiconductor detectors is that when the intention is to measure p (protons), d (deuterons), t (tritons) and a particles, the background radiation containing neutrons and gamma particles should be eliminated. Conventional semiconductor silicon detectors are not sensitive enough to distinguish between the desired particles and the neutrons and gamma particles since they give more energy to the detected signal than the intended measurements. There are many kinds of different particles to be detected in plasma. When there are intense gamma and neutron backgrounds it is therefore difficult to identify different types of incident particles created in the plasma.
The object of the invention is a detector with which high-intense radiation can be measured and which is sensitive enough for distinguishing signals caused by p (protons), d (deuterons), t (tritons) and a particles, form background radiation containing neutrons and gamma particles.